Method and apparatus for handing over a subscriber unit between celluar communication systems

ABSTRACT

The invention relates to handover of a subscriber unit ( 301,305 ) from a first cellular communication system ( 201 ) to a second cellular communication system ( 203 ). The first communication system is for example a UMTS communication system supporting a plurality of connections, and the second communication system is for example a GSM communication system supporting only a single connection. The handover comprises forming a handover connection  219  to the subscriber unit ( 301,305 ) through the second communication system and handing one of the connections under the first communication system over to this connection. Rather than dropping the remaining connections, they are entered into a holding state. After the handover, one of the connections on hold may be entered into an active state by associating the handover connection ( 219 ) with this connection.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for handing over asubscriber unit between cellular communication systems and in particularto handovers from a third generation cellular communication system to asecond generation cellular communication system.

BACKGROUND OF THE INVENTION

FIG. 1 illustrates the principle of a conventional cellularcommunication system 100 in accordance with prior art. A geographicalregion is divided into a number of cells 101, 103, 105, 107 each ofwhich is served by base station 109, 111, 113, 115. The base stationsare interconnected by a fixed network which can communicate data betweenthe base stations 101, 103, 105, 107. A mobile station is served via aradio communication link by the base station of the cell within whichthe mobile station is situated. In the example if FIG. 1, mobile station117 is served by base station 109 over radio link 119, mobile station121 is served by base station 111 over radio link 123 and so on.

As a mobile station moves, it may move from the coverage of one basestation to the coverage of another, i.e. from one cell to another. Forexample mobile station 125 is initially served by base station 113 overradio link 127. As it moves towards base station 115, it enters a regionof overlapping coverage of the two base stations 111 and 113 and withinthis overlap region it changes to be supported by base station 115 overradio link 129. As the mobile station 125 moves further into cell 107,it continues to be supported by base station 115. This is known as ahandover or handoff of a mobile station between cells.

A typical cellular communication system extends coverage over typicallyan entire country and comprises hundred or even thousands of cellssupporting thousands or even millions of mobile stations. Communicationfrom a mobile station to a base station is known as uplink, andcommunication from a base station to a mobile station is known asdownlink.

Currently the most ubiquitous cellular communication system is the2^(nd) generation communication system known as the Global System forMobile communication (GSM). GSM uses a technology known as Time DivisionMultiple Access (TDMA) wherein user separation is achieved by dividingfrequency carriers into 8 discrete time slots, which individually can beallocated to a user. A base station may be allocated a single carrier ora multiple of carriers. One carrier is used for a pilot signal whichfurther contains broadcast information. This carrier is used by mobilestations for measuring of the signal level of transmissions fromdifferent base stations, and the obtained information is used fordetermining a suitable serving cell during initial access or handovers.Further description of the GSM TDMA communication system can be found in‘The GSM System for Mobile Communications’ by Michel Mouly and MarieBernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

Currently, 3^(rd) generation systems are being rolled out to furtherenhance the communication services provided to mobile users. The mostwidely adopted 3^(rd) generation communication systems are based on CodeDivision Multiple Access (CDMA) wherein user separation is obtained byallocating different spreading and scrambling codes to different userson the same carrier frequency. The transmissions are spread bymultiplication with the allocated codes thereby causing the signal to bespread over a wide bandwidth. At the receiver, the codes are used tode-spread the received signal thereby regenerating the original signal.Each base station has a code dedicated for a pilot and broadcast signal,and as for GSM this is used for measurements of multiple cells in orderto determine a serving cell. An example of a communication system usingthis principle is the Universal Mobile Telecommunication System. (UMTS),which is currently being deployed. Further description of CDMA andspecifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley &Sons, 2001, ISBN 0471486876.

In a UMTS CDMA communication system, the communication network-comprises a core network and a Radio Access Network (RAN). The corenetwork is operable to route data from one part of the RAN to another,as well as interfacing with other communication systems. In addition, itperforms many of the operation and management functions of a cellularcommunication system, such as billing. The RAN is operable to supportwireless user equipment over a radio link being part of the airinterface. The RAN comprises the base stations, which in UMTS are knownas Node Bs, as well as Radio Network Controllers (RNC) which control theNode Bs and the communication over the air interface.

Whereas the GSM system was originally developed with a view to mainlysupporting voice services, UMTS has from the outset been developed toprovide a wide plurality of different services including different voicecommunication services and data services. Different services can be setup having different characteristics and specifically with differentQuality of Service parameters such as different delays, data rates,target error rates etc. Further, UMTS provides for a plurality ofservices and connections to be set up between different mobile stations.

The deployment of UMTS will initially be in islands of coverage whereinUMTS base stations provide coverage of selected areas and regions.However, it is expected that most UMTS systems will not provide full orextensive coverage for some time, and therefore it is planned that thegaps in UMTS coverage will be covered by 2^(nd) Generation Systems suchas GSM. Consequently, multimode mobile stations capable of operatingwith both GSM and UMTS are being developed as is handover methodsbetween the two systems.

However, GSM provides limited services in comparison to UMTS and thehandover methods therefore comprise reducing the services and quality ofservice provided to a user when handing over to GSM. Hence, the handovercauses a reduced service level for a user therefore a system forhandovers providing improved support of services, functionality and/orperformance would be an advantage.

SUMMARY OF THE INVENTION

Accordingly the Invention seeks to mitigate, alleviate or eliminate oneor more of the above mentioned disadvantages singly or in anycombination.

Accordingly, there is provided a method of handing over a subscriberunit from a first cellular communication system supporting a pluralityof connections of the subscriber unit to a second cellular communicationsystem; the method comprising the steps of: entering at least a firstconnection of said plurality of connections into a holding state;forming a handover connection to the subscriber unit through the secondcommunication system; handing over a second connection of said pluralityof connections to the second cellular communication system byassociating the second connection with said handover connection;entering said at least first connection into an active state byassociating the at least first connection with the handover connection.

An advantage provided by the invention is that upon handover from afirst to a second communication system, the handover can be performedfor just a single connection while still maintaining all connections ofthe first communication system. This is especially advantageous when thesecond communication system is only able to support one connection, asit allows for the remaining connections to be maintained rather thandropped. The connections put into a holding state can be put into anactive state using the handover connection and thus without requiringmore than one connection in the second cellular communication system. Ifthe subscriber unit is subsequently handed back to the UMTScommunication system, it allows for the connections currently in aholding state to immediately be re-established. Hence, an improvedservice is provided to the subscriber units in a dual cellularcommunication system which efficiently accommodates for the differentcapabilities, services and performance of the different communicationsystems.

According to a different feature of the invention, the step of enteringsaid at least first connection into an active state comprises switchingthe handover connection from being with the second connection to beingwith the at least first connection. Switching between connectionsprovide for a simple and efficient implementation suitable for cellularcommunication systems.

According to another feature of the invention, the method furthercomprises the step of selecting the second connection from the pluralityof connections in response to at least one characteristic of at leastone of the plurality of connections. Advantageously, the connectionhaving a characteristic most suited for being associated with thehandover connection can thus be selected as the second connection.

According to another feature of the invention, the characteristic isrelated to a type of connection and preferably the step of selectingcomprises selecting a data service connection in preference to a voiceservice connection as the second connection. This provides the advantageof minimising the service impact by putting connections into a holdingstage as a voice service typically can be continued after the connectionhas been entered into the active state without unacceptableinconvenience.

According to another feature of the invention, the at least onecharacteristic comprises at least one characteristic chosen from thegroup consisting of a priority; a data rate; a propagationcharacteristic; an error rate; a transaction identifier; and a time ofsetup of at least one of the plurality of connections. These parametersprovide efficient and practical parameters for selection the secondconnection.

According to another feature of the invention, the at least firstconnection is a data connection and the method comprises the steps ofstoring data of the at least first connection in memory when the atleast first connection is in the holding state; and communicating thedata stored in said memory when the at least first connection enters theactive state. This allows for a data service to be put on hold withoutany loss of data. The data source for the data service need not stop thedata transmission or be informed of the holding state as the datacommunicated is buffered and forwarded to the subscriber unit when thedata connection enters the active state.

According to another feature of the invention, the at least firstconnection is a data connection and the method comprises the steps ofstoring data of the at least first connection in memory when the atleast first connection is in the holding state; and the subscriber unitretrieving the stored data from the memory by setting up a separate datacall. This allows for a data service to be put on hold without any lossof data. The data source for the data service need not stop the datatransmission or be informed of the holding state as the datacommunicated is stored and retrieved by the subscriber unit. The user ofa subscriber unit may have control of when to receive the stored dataand this does not require an active connection.

According to another feature of the invention, the method furthercomprises the step of notifying a user of the subscriber unit of whichof the plurality of connections are in a holding state. Thereby the useris informed of the current status and thus may take appropriate action.

According to another feature of the invention, at least one of theplurality of connections is between the subscriber unit and a secondcommunication unit and the method further comprises the step ofnotifying a user of the second communication unit of which of theplurality of connections are in a holding state. Advantageously, theuser of the second communication system is informed of the currentstatus and thus may take appropriate action

According to another feature of the invention, the notification is bymeans of a voice communication if at least one of the plurality ofconnections is a voice service connection. This is provides an efficientmeans for notification which is easily interpreted by a user.

According to another feature of the invention, the step of selecting thefirst connection is in response to a parameter set by an operator of atleast one of the first or second cellular communication systems.Advantageously, the operator may control parameters influencing theselection of the second connection thereby allowing the operatorincreased control over the communication systems and services provided.

According to another feature of the invention, the step of selecting thesecond connection is in response to a parameter set by a user of the.subscriber unit. Advantageously, the user is thus provided with meansfor influencing which connection should initially be in an active state.

According to another feature of the invention, if the handover to thesecond cellular communication system is unsuccessful at least one of theplurality of connections is re-established through the firstcommunication system. Hence, the advantage of increased resilience andreduced dropped connection rates is achieved as a connection is onlydropped if the handover to the second communication system fails and nosupport for the connection is available in the first communicationsystem.

According to another feature of the invention, the second cellularcommunication system comprises a master switch center comprisingfunctionality for selecting the second connection out of the pluralityof connections. This provides for a very efficient implementationsuitable for most cellular communication systems.

According to another feature of the invention, the method is operated ina single integrated master switch centre for the first cellularcommunication system and the second cellular communication system. Thisprovides for the advantage of allowing for an efficient and relativelylow complexity implementation wherein the characteristics andfunctionality of both communication systems can be considered andutilised.

The second communication system may be operable to only support oneconnection for each served subscriber unit and the connections arepreferably circuit switched connections.

Preferably, the first cellular communication system is a ThirdGeneration Cellular Communication System such as a Universal MobileTelecommunication System (UMTS).

Preferably the second cellular communication system is a SecondGeneration Cellular Communication System such as the Global System forMobile communication (GSM) cellular communication system.

According to a second aspect of the invention, there is provided anapparatus for handing over a subscriber unit from a first cellularcommunication system supporting a plurality of connections of thesubscriber unit to a second cellular communication system; the apparatuscomprising: means for entering at least a first connection of saidplurality of connections into a holding state; means for forming ahandover connection to the subscriber unit through the secondcommunication system; means for handing over a second connection of saidplurality of connections to the second cellular communication system byassociating the second connection with said handover connection; meansfor entering said at least first connection into an active state byassociating the at least first connection with the handover connection.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will be described, by way of exampleonly, with reference to the drawings, in which

FIG. 1 is an illustration of a cellular communication system inaccordance with the prior art;

FIG. 2 is an illustration of an architecture of two cellularcommunication systems to which an embodiment of the invention may apply;and

FIG. 3 is an illustration of handover situation to which a preferredembodiment of the invention may be applied.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The invention will in the following be described with specific referenceto handovers between the UMTS communication system standardised by theThird Generation Partnership Project (3GPP) and the GSM communicationsystem standardised by the European Telecommunication StandardsInstitute (ETSI), but it will be apparent that the invention is notlimited to this application but is equally applicable to many othercommunication systems and handovers.

FIG. 2 is an illustration of architectures of cellular communicationsystems 201, 203 to which an embodiment of the invention may apply.

A first communication system 201 is a UMTS communication systemsupporting a plurality of UMTS subscriber units. The secondcommunication system 203 is a GSM communication system supporting aplurality of GSM subscriber units. In addition both communicationsystems 201, 203 are able to support a number of multi mode subscriberunits comprising both GSM and UMTS functionality. A subscriber unit maytypically be a wireless user equipment, a mobile station, acommunication terminal, a personal digital assistant, a laptop computer,an embedded communication processor or any communication elementcommunicating over the air interface.

The UMTS communication system comprises a UMTS Mobile Switch Centre(MSC) 205 which among other things is in charge of switching theincoming data streams from different subscriber units to the appropriateoutput streams for the call destination. Specifically, the MSC 205 mayswitch the data to another MSC. Further, the MSC 205 is, in the exampleshown, a Gateway—MSC comprising a gateway interface for interfacing toan external network which in this case is a Public Standard TelephoneNetwork (PSTN) 207. The MSC further comprises functionality forperforming various maintenance and operational functions includingauthentication and billing.

The UMTS MSC 205 is connected to a plurality of UMTS Radio Networkcontrollers (RNCs) 209 of which one is shown in FIG. 2. The RNC 209 isconnected to a number of UMTS base stations, which are known as Node Bs211, 213, of which two are shown in FIG. 2. The RNC 209 performs many ofthe control functions related to the air interface of the UMTScommunication system 201 including radio resource management and routingof data to and from appropriate Node Bs 211, 213. An RNC and associatedNode Bs is known as a Radio Network System (RNS).

The Node Bs 211,213 comprise much of the functionality required forsupporting subscriber units over the radio air interface. As such, theNode Bs comprise radio transmitters, receivers and the required controlcircuitry for communicating with a plurality of subscriber units. FIG. 2illustrates three UMTS subscriber units 215, 217, 219 communicating withone of the Node Bs 213. The communication is by means of radiocommunication in accordance with the Technical Specifications for theUMTS communication system. Typically, each Node B will support a highnumber of subscriber units simultaneously.

The GSM communication system 203 comprises a GSM Mobile Switch Centre(MSC) 221 which generally performs the same functions in the GSMcommunication system 203 as the UMTS MSC 205 performs for the UMTScommunication system 201. It thus provides switching, operations andmaintenance functions as well as interfacing to external communicationsystems.

The GSM MSC 221 is connected to a GSM Base Station Controller (BSC) 223which provides control functionality for the base stations includingchannel allocation, channel configuration management and handovercontrol. The BSC 223 is connected to a plurality of GSM base stations225, 227 known as Base Transceiver Stations (BTSs) in GSM. The BTSs 225,227, of which two are shown in FIG. 2, comprise the functionalityrequired for communicating over the GSM air interface to GSM subscriberunits including the required radio transmitters, receivers and controlfunctionality. In the example of FIG. 2, each of the illustrated BTSs225, 227 are shown to communicate with a single GSM subscriber unit229,231, but typically each BTS 225, 227 simultaneously communicate witha high number of subscriber units.

In the example of FIG. 2, one of the UMTS subscriber units 219 is a dualmode subscriber unit which is operable to communicate according to boththe UMTS and the GSM communication standards. If the dual modesubscriber unit 219 moves out of the coverage area of the supportingNode B 213 it will attempt to handover to another Node B, and thusremain on the UMTS communication system 201. However, it may bepreferred, for example because the dual mode subscriber unit 219 ismoving into an area with insufficient UMTS coverage, to handover to aGSM BTS. In this case, it will be attempted to set up a suitablecommunication link between the dual mode subscriber unit 219 and a GSMBTS 225 as illustrated by the connection 233 in FIG. 2.

However, as the two communication systems 201, 203 are not identicalthey have different characteristics and capabilities. In particular, theGSM cellular communication system 203 does not provide the same varietyand performance of services as can be achieved in a UMTS communicationsystem 201. Specifically, it is possible to set up a plurality ofconnections to a subscriber unit in a UMTS cellular communicationsystem. Hence, in UMTS, each connection is terminated in the subscriberunit. The plurality of connections may have different characteristics,and thus different services may simultaneously be set up for asubscriber unit including for example a mix of voice and data servicesthat may further have different characteristics. However, in GSM onlyone physical connection is allowed for each subscriber unit on theinterface between the GSM MSC 221 and the BSC 223, and therefore onlyone simultaneous connection is set up to the dual mode subscriber unit219 through the GSM communication system 203.

In accordance with a preferred embodiment of the invention, a subscriberunit 219 having a plurality of associated connections is handed overfrom a first cellular communication system, such as the UMTScommunication system 201, to a second cellular communication system,such as the GSM communication system 203, by the performing the steps ofentering at least a first connection of said plurality of connectionsinto a holding state; forming a handover connection to the subscriberunit 219 through the second communication system 203; handing over asecond connection of said plurality of connections to the secondcellular communication system 203 by associating the second connectionwith said handover connection; and entering the at least firstconnection into an active state by associating the at least firstconnection with the handover connection.

FIG. 3 is an illustration of handover situation to which a preferredembodiment of the invention may be applied. For clarity and brevity,only the elements of the cellular communication systems of FIG. 2 whichare directly involved in supporting a specific subscriber unit areshown. In the example of FIG. 3, a subscriber unit 301 is communicatingover three circuit switched connections with a second communication unit303. The second communication unit 303 is in this example a PSTNcommunication unit, which is connected to the UMTS cellularcommunication system through the PSTN Gateway 207. The secondcommunication unit 303 may thus for example be a bridge conferencesystem or in the specific example may be a network unit of the PSTNsupporting three separate telephone connections.

It will be apparent, that the second communication unit 303 could be anysuitable communication unit and could be associated with the cellularcommunication system in any suitable way. As such, the secondcommunication unit could be a communication unit associated with adifferent external network, such as the internet, or could be associatedwith the first communication system, such as for example a second UMTSsubscriber unit. Further, it will be apparent that instead of a singlesecond communication unit, the subscriber unit 301 could be incommunication with a plurality of communication units, and that thesefurther could be different communication units having differentcharacteristics, capabilities and associations with the cellularcommunication system(s). As such, the second communication unit 303could be associated with the second communication system.

In the specific example of FIG. 3, the subscriber unit 301 iscommunicating through three circuit switched voice connections. Hence,the subscriber unit 301 is simultaneously supporting three voiceservices. For example, the subscriber unit 301 may in this case be aconference bridge unit that maintains a conference between four usersthrough the three connections. In order to support the three circuitswitched connections, at least three connections are established for thesubscriber unit 301 between the subscriber unit 301 and the serving NodeB 213, between the serving Node B 213 and the serving RNC 209, betweenthe serving RNC 209 and the UMTS MSC 205, between the UMTS MSC 205 andthe PSTN Gateway 207 and between the PSTN gateway 207 and the PSTNcommunication unit 303.

In the present example, the subscriber unit 301 moves out of thecoverage area of the UMTS communication system 201. In the illustratedexample, the subscriber unit 301 is a dual mode subscriber unit andtherefore it may be attempted to set up a communication with the GSMcommunication system 203. For this reason, the subscriber unit 301measures the pilot signals (such as the BCCH broadcast transmissions) ofthe neighbouring GSM BTSs in the same way as a conventional GSM mobilestation does. In addition, the subscriber unit 301 measures beaconsignals of the neighbouring UMTS Node Bs. These measurements arereported back to the UMTS cellular communication system, wherein it isdetermined if a handover is necessary, and if so to which base stationthe handover should be directed to. If the reported measurementsindicate that no suitable UMTS handover candidate is available but thatthe subscriber unit may be supported by a GSM BTS, a handover is made tothis BTS. The selection of which base station to handover to ispreferable determined in the same manner as for a standard GSM mobilestation. Specifically the base station being reported with the highestmeasured BCCH signal level is chosen.

When a GSM base station has been selected, the necessary signalling andinformation is exchanged between the communication systems and thesubscriber unit, within each of the communication systems and betweenthe communication systems. Specifically, in the preferred embodiment,the handover controller determining whether to handover to the GSMcommunication system is located in the UMTS cellular communicationsystem 201, whereas the handover controller for determining which GSMBTS to handover to is located in the GSM communication system 203. Inthis case, the subscriber unit 301 first reports the measurement valuesto the RNC 209. The RNC 209 processes the measurements together withother characteristics to determine whether a handover is necessary andwhether such a handover should be to the GSM communication system 203.In this case, a handover request is transmitted to the UMTS MSC 205 andfrom there to the GSM MSC 221. The GSM MSC 221 processes the informationand selects the suitable GSM BTS 225. It then communicates thecharacteristics of this BTS 225 back to the RNC 209 in the UMTScommunication system 201. The RNC 209 forwards the necessary informationto the subscriber unit 301 through the Node B 213. Upon receiving theinformation, the subscriber unit 301 proceeds to access the GSMcommunication system 203 by transmitting access messages over the GSMair interface. The GSM cellular communication 201 system then forms aGSM communication link with base station. This is in FIG. 3 illustratedby the subscriber unit 305 which represents the subscriber unit 301following successful handover to the GSM communication system 203. Thusin FIG. 3, the subscriber units 301 and 305 represents the same dualmode subscriber unit before and after handover, i.e. in the UMTS modeand GSM mode respectfully. Once the GSM communication link has beensuccessfully set up, the UMTS communication system 201 is informed, andit then proceeds to terminate the UMTS connections to the subscriberunit 301.

It will be appreciated that the specific embodiment described above ismerely an example of how a handover could be implemented. For brevityand clarity, only a general implementation possibility has beendescribed, and in practical UMTS and GSM communication systems othermethods and signal exchanges may be used.

It will thus be apparent that any suitable method for determining andselecting that a handover is required, which communication system tohand over to, and which base station of that communication system tohand over to may be used without detracting from the invention. Likewiseany suitable method of implementing the handover method may be usedincluding any suitable signalling protocol, distribution of handoverfunctionality and access mechanism.

However, as illustrated in FIG.3, the communication link of the GSMcommunication system 203 only supports one circuit switch connection.Conventionally, handover from a UMTS communication system is thereforeassociated with handover of just one connection while the remainingconnections are released or dropped.

In a preferred embodiment of the current invention, all the connectionssupported by the UMTS communication system 201 are forwarded to the GSMcellular communication system 203. Thus as illustrated in FIG. 3, in thedescribed example three connections are established between the UMTS MSC205 and the GSM MSC 221, one for each of the connections from the PSTNcommunication unit 303. However, from the GSM MSC 221 through the BSC223 to the BTS 225 only one connection is established.

In the described embodiment, the GSM MSC 221 thus sets up a singlehandover connection from the GSM MSC 221 to the subscriber unit 305. Itthen performs a handover by switching this handover connection to one ofthe connections from the UMTS MSC 205. Further, the GSM MSC 221comprises functionality for entering the remaining connections into aholding state. In the holding state, the connection is maintained to thesecond communication unit 303 but no data is communicated to thesubscriber unit 305. For a voice call, the holding state may simplycomprise putting the call on hold, which may be achieved by transmittinga holding indication to the second communication unit thereby informingthe user of that connection that the call is on hold.

Following the inter-system handover, the subscriber unit 305 is thusprovided with a single connection out of the plurality of originalconnections. However, in contrast to conventional methods, the remainingconnections are not dropped but are still maintained. The GSM MSC 221further comprises functionality for switching the handover connectionbetween the GSM MSC 221 and the subscriber unit 305 to one of theconnections on hold. Thus, following the handover, a connection in aholding state may be entered into an active state by switching thehandover connection from the active connection to a connection on holdthereby allowing communication to the subscriber unit 305 from thisconnection. The previously active connection is preferably put into aholding state.

Hence, in accordance with the preferred embodiment, the handover methodallows all connections supported in the UMTS cellular communicationsystem to be maintained when handing over to a GSM cellularcommunication system. Communication can be maintained for allconnections by switching a single GSM connection between the pluralityof connections. A significant advantage of this approach, is that if thesubscriber unit subsequently performs a handover back to the UMTScommunication system, all the original connections can readily bere-established.

It will be appreciated that the above description merely sets out aspecific example embodiment, and that many variations and modificationsmay be made without detracting from the invention. For example, althoughthe association between the plurality of UMTS connections and the oneGSM handover connection is preferable made by a simple switchingoperation, other embodiments may use more complex methods includingoperating a queue allocation scheme or multiplexing scheme.

In the preferred embodiment, the selection of which connection out ofthe plurality of connections to handover is in response to at least onecharacteristic of one of the connections. Specifically, thecharacteristic is related to a type of connection of the plurality ofconnections. Thus, the GSM MSC 221 selects the connection to initiallyassociate with the handover connection from a consideration of whichtypes of connections that are comprised in the plurality of connectionsbeing supported by the UMTS communication system. In the preferredembodiment, this consideration includes considering whether any of theconnections are data service connections and if so selecting a dataservice connection in preference to a voice service connection as theconnection initially being associated with the handover connection. Inmost communication systems, there is no provision for pausing a datasource for a data service whereas a voice call can be put on hold by asimple notification of the user. Thus for a voice call, an interruptionin the communication is acceptable if the users are notified thereof,whereas for a data call an interruption in the connection through thecommunication system(s) will cause loss of data. Hence, by prioritisingdata service connections above voice service connections, the negativeimpact of handing a plurality of connections over to a single connectionis reduced.

Additionally or alternatively, the selection of the initial connectionfor the handover can be in response to one or more of the followingparameters:

A priority of a connection: The communication system may have means ofprioritising different connections, and the selection of the initiallyactive connection may be determined as the one having the highestpriority. The priority may be determined directly by a priorityparameter or may be determined indirectly by evaluation of otherparameters. For example, quality of service parameters for theconnections may indicate how highly they are prioritised. For example,data connections having strict error and delay requirements may beprioritised higher than data connections having less strictrequirements.

A data rate: The initial connection may be selected in accordance withthe data rate of the service supported by a connection. The connectionmay simply be selected as the connection having the highest data ratebut in the preferred embodiment, it is selected as the data rate whichmost closely corresponds to the data rate that is provided by thehandover connection. Thus the initially active connection will beselected as the connection that will be the least affected by thehandover.

An error rate: The selection of the initially active connection may forexample be in response to an error rate of the service of theconnection. This error rate may both be a required one or a measurederror rate. Preferably, the connection is selected as that having arequired error rate that most closely corresponds to a current errorrate measured for the connection over the GSM air interface.

A transaction identifier: A transaction identifier is used by layer 3 ofthe UMTS communication system to identify different connections andtransactions. This identification number may directly be used to selectan initial connection, for example by selecting the connection that hasthe lowest transaction identifier.

A time of setup of at least one of the plurality of connections: Thusthe selection of the initially active connection may be in response tothe time of setting up the connections. Specifically, the initiallyactive connection may be selected as the first connection that was setup for the communication system. In other embodiments, the initiallyactive connection may be selected as the one most recently having beenset up.

In some embodiments, the selection of the initially active connection isin response to a parameter set by an operator of at least one of thefirst or second cellular communication systems. Thus, a network operatormay control the handover mechanism by for example establishing aprioritised list of parameters in response to which the initialconnection is selected. An example of such a list would be to firstchoose between connections depending on whether they are data serviceconnections or voice service connections. If there is more than one dataservice connections, the operator may set up selection between these tobe in response to the data rate such that the connection having a datarate most closely resembling the one of the handover connection isselected.

In other embodiments, the selection of the initially active connectionis additionally or alternatively in response to a parameter set by auser of the subscriber unit. Hence, in this embodiment the user isdirectly involved in the selection of the initially active connectionthereby allowing the user control over which connections are put on holdand which connection remains active. In a simple embodiment, the user issimply presented with a list of available connections and asked toselect one of these.

In accordance with an embodiment of the invention, at least one of theconnections entered into a holding state is a data connection supportinga data service and specifically a circuit switched data service. In thisembodiment, the method of handover further comprises storing data of theat least first connection in memory when the at least first connectionis in the holding state; and communicating the data stored in saidmemory when the at least first connection enters the active state.Hence, in this embodiment, the MSC further comprises a suitable datastorage memory. As the data connection is put into the holding state,the data received from the UMTS communication system is stored in thedata storage memory. Consequently, when the handover connection isswitched to the data connection, the stored data may be downloaded tothe subscriber unit through the handover connection. In accordance withthis embodiment, data buffering is thereby provided resulting in no databeing lost during the handover. Hence, the operation of the originatingdata source is not affected by any handovers between the differentcommunication systems. This embodiment is particularly suited for delayinsensitive data services such as for example email services.

Additionally or alternatively, the subscriber unit may retrieve thestored data from the memory by setting up a separate data call. Thus, inthis embodiment, the data from the data connection is stored and can beretrieved independently of the current connections. Consequently, if thesubscriber unit has capability therefor, it may set up a differentindependent connection over the GSM air interface to the GSM MSC anddownload the data from the storage means. Thereby, the subscriber unithas control over the download of the buffered data, and it can retrievethe stored data while another of the plurality of connections is activevia the handover connection.

In the preferred embodiment, the handover method comprises the step ofnotifying a user of the subscriber unit of which of the plurality ofconnections are in a holding state. Additionally or alternatively, thehandover method comprises the step of notifying a user of the secondcommunication unit of which of the plurality of connections are in aholding state. Thus when a connection is put on hold, a notificationsignal is communicated to the originating communication unit(s)informing it (them) that the connection is put on hold. An originatingcommunication unit is thus informed that real time communication withthe subscriber unit is not currently available. The originatingcommunication unit or a user thereof may thus take appropriate action,such as for example pausing the transmission. In a simple embodiment,where the connection supports a voice service, the user of theoriginating communication unit may simple pause his speech until theconnection is entered into an active state. Thus a notification of aconnection entering an active state is also preferably transmitted toone or both of the subscriber unit and the originating communicationunit.

Likewise, the subscriber unit or a user thereof may upon notificationthat one or more of the connections are put on hold take appropriateaction. In the simple case of voice communications, the user of thesubscriber unit will simply refrain from speaking to the party of aconnection put on hold. Hence, in the simple case of the subscriber unitacting as a conference bridge, only one voice connection will be activeat a time when supported by the GSM communication system. The he user ofthe subscriber unit will proceed by only speaking to the party of thecurrently active voice call. However, in the preferred embodiment, theuser of the subscriber unit is further provided an indication of thevoice calls currently on hold and with with switching means allowing himto manually select which call is to be in the active state. Thus, themethod of the preferred embodiment comprises switching of the handoverconnection between the plurality of subscriber units in response to auser input of a user of the subscriber unit. Hence, the user maycontinue to communicate with all parties by sequentially switchingbetween the connections available. In embodiments where the connectionssupport voice services, the notification is preferably by means of avoice communication, such as for example a synthesized voice messageindicating which connections are on hold.

In accordance with one embodiment of the invention, the method furtherdetermines if the handover to the second cellular communication system,such as the GSM cellular communication system described above, isunsuccessful. If so, at least one of the plurality of connections isre-established through the first communication system, such as the UMTScommunication system described above. Hence, if the handover to the GSMcommunication system is not successful, the UMTS RNC 209 will attempt tomaintain and re-establish the connection through the UMTS communicationsystem. This may be through the previously serving Node B 213 or throughanother Node B. Hence, in this embodiment, a call is not dropped becausea connection to the GSM communication system cannot be established butonly if additionally the connection cannot be re-established through theUMTS communication system.

It will be apparent that the different functionality may be implementedin any suitable physical, logical and structural location. As such, theassociation between the handover connection and one of the plurality ofconnections may be made in the first or second communication system.However, preferably the association comprises a switching functionimplemented in the GSM MSC. In the preferred embodiment, the GSM MSCfurther comprises the functionality for selecting the initially activeconnection.

In the preferred embodiment, a connection that has been establishedunder one MSC such as the UMTS MSC, remains under the control of thatMSC following a handover. The original MSC is thus an anker MSC, whichcontinues to be responsible for gathering and processing billing data aswell as the signalling and overall call establishment and maintenance.In the preferred embodiment, the MSCs of the two cellular communicationsystems are furthermore an integrated MSC comprising both the UMTS andGSM functionality. In this embodiment, the interface between the twocommunication systems is thus implicitly implemented in thefunctionality of the integrated MSC. This allows for a high degree ofinteraction between the communication systems and for a very efficienthandover processing.

In the preferred embodiment the first cellular communication system isthus preferably a Third Generation Cellular Communication System such asUMTS, and the second communication system is a Second GenerationCellular Communication System, such as GSM. Therefore the operation ofthe handover is in accordance with the technical specifications of theUMTS and GSM communication respectively. In particular the entering ofone or more of the connections into a holding state is performed inaccordance with the 3^(rd) Generation Partnership Project (3G PP)Technical Specifications 22.083, 23.083 and/or 24.083, and the enteringof one of these connections into an active state is performed inaccordance with the 3^(rd) Generation Partnership Project (3G PP)Technical Specification 24.083. However, it will be cleat that in otherembodiments, other communication systems may be involved and otherstandards may be adhered to.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. However,preferably, the invention is implemented as computer software running onone or more data processors. The elements and components of anembodiment of the invention may be located in the core network, theradio access network or any suitable physical or functional location.Indeed the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, theinvention may be implemented in a single unit or may be physically andfunctionally distributed in the network.

Although the present invention has been described in connection with thepreferred embodiment, it is not intended to be limited to the specificform set forth herein. Rather, the scope of the present invention islimited only by the accompanying claims.

1. A method of handing over a subscriber unit from a first cellularcommunication system supporting a plurality of connections of thesubscriber unit to a second cellular communication system (havingcapability for supporting only one connection), the method comprisingentering at least a first connection of said plurality of connectionsinto a holding state; forming a handover connection to the subscriberunit through the second cellular communication system; handing over asecond connection of said plurality of connections to the secondcellular communication system by associating the second connection withsaid handover connection; entering said at least first connection intoan active state by associating the at least first connection with thehandover connection:
 2. A method as claimed in claim 1 wherein enteringsaid at least first connection into an active state comprises switchingthe handover connection from being with the second connection to beingwith the at least first connection.
 3. A method as claimed in claim 1further comprising selecting the second connection from the plurality ofconnections in response to at least one characteristic of at least oneof the plurality of connections.
 4. A method as claimed in claim 3wherein the characteristic is related to a type of connection.
 5. Amethod as claimed in claim 4 wherein selecting comprises selecting adata service connection in preference to a voice service connection asthe second connection.
 6. A method as claimed in claim 3 wherein the atleast one characteristic comprises at least one characteristic chosenfrom the group consisting of a) a priority; b) a data rate; c) apropagation characteristic; d) an error rate; e) a transactionidentifier; and f) a time of setup of at least one of the plurality ofconnections.
 7. A method as claimed in claim 1 wherein the at leastfirst connection is a data connection and the method comprises storingdata of the at least first connection in memory when the at least firstconnection is in the holding state; and communicating the data stored insaid memory when the at least first connection enters the active state.8. A method as claimed in claim 1 wherein the at least first connectionis a data connection and the method comprises storing data of the atleast first connection in memory when the at least first connection isin the holding state; and the subscriber unit retrieving the stored datafrom the memory by setting up a separate data call.
 9. A method asclaimed in claim 1 further comprising notifying a user of the subscriberunit of which of the plurality of connections are in a holding state.10. A method as claimed in claim 1 wherein at least one of the pluralityof connections is between the subscriber unit and a second cellularcommunication unit and further comprising notifying a user of the secondcellular communication unit of which of the plurality of connections arein a holding state.
 11. A method as claimed in claim 9 wherein thenotification is by means of a voice communication if at least one of theplurality of connections is a voice service connection.
 12. A method asclaimed in claim 1 further comprising selecting the second connection inresponse to a parameter set by an operator of at least one of the firstor second cellular communication systems.
 13. A method as claimed inclaim 1 further comprising selecting the second connection in responseto a parameter set by a user of the subscriber unit.
 14. A method asclaimed in claim 1 wherein if the handover to the second cellularcommunication system is unsuccessful at least one of the plurality ofconnections is re-established through the first cellular communicationsystem.
 15. A method as claimed in claim 1 wherein the second cellularcommunication system comprises a master switch center comprisingfunctionality for selecting the second connection out of the pluralityof connections.
 16. A method as claimed in claim 1 wherein the method isoperated in a single integrated master switch centre for the firstcellular communication system and the second cellular communicationsystem.
 17. A method as claimed in claim 1 wherein the second cellularcommunication system is operable to only support one connection for eachserved subscriber unit.
 18. A method as claimed in claim 1 wherein theplurality of connections is circuit switched connections.
 19. A methodas claimed in claim 1 wherein the second cellular communication systemis a Second Generation Cellular Communication System.
 20. A method asclaimed in claim 19 wherein the second cellular communication system isa Global System for Mobile communication (GSM) cellular communicationsystem.
 21. A method as claimed in claim 1 wherein the first cellularcommunication system is a Third Generation Cellular CommunicationSystem.
 22. A method as claimed in claim 21 wherein the first cellularcommunication system is a Universal Mobile Telecommunication System(UMTS).
 23. A method as claimed in claim 22 wherein entering the atleast first connection into a holding state is performed in accordancewith at least one of the 3^(rd) Generation Partnership Project (3G PP)Technical Specifications 22.083, 23.083 and 24.083.
 24. A method asclaimed in claim 22 wherein entering said at least first connection intoan active state is performed in accordance with the 3^(rd) GenerationPartnership Project (3G PP) Technical Specification 24.083.
 25. Aapparatus for handing over a subscriber unit from a first cellularcommunication system supporting a plurality of connections of thesubscriber unit to a second cellular communication system (havingcapability for supporting only one connection); the apparatuscomprising: means for entering at least a first connection of saidplurality of connections into a holding state; means for forming ahandover connection to the subscriber unit through the second cellularcommunication system; means for handing over a second connection of saidplurality of connections to the second cellular communication system byassociating the second connection with said handover connection; meansfor entering said at least first connection into an active state byassociating the at least first connection with the handover connection.